Multi-channel grating structures are typically written into photosensitive waveguides. A typical multi-channel grating structure comprises variations in the refractive index of the photosensitive waveguide over the dimensions of the photosensitive waveguide. The refractive index variations are induced by exposing the waveguide to an appropriate pattern of radiation, such as UV light.
Various applications for waveguides require multi-channel grating structures with different spectral characteristics, such as reflection, transmission and group delay characteristics. The profile of refractive index variation over the dimensions of the waveguide determines the optical characteristics of the multi-channel grating structure. A design process is used to analytically or numerically obtain a profile of refractive index variation which, when written to a photosensitive waveguide, results in a grating structure with the desired spectral characteristics.
The materials used to produce photosensitive waveguides are capable of only a limited maximum photo-induced refractive index change. As a result, when designing a multi-channel grating structure, a challenge is to develop a multi-channel grating structure that, on one hand, satisfies the desired spectral characteristics and, on the other hand, has as small as practically possible peak of the refractive index variation (apodisation profile).
One approach to designing multi-channel grating structures is to use periodic sampling. The periodic sampling design method involves the periodic sampling of a single grating structure to produce a multi-channel grating structure. The sampling process involves periodic modulation of the amplitude and/or phase of the single-channel grating structure in the spatial domain and is analogous to the repeat superposition of grating structures. The periodic sampling method produces a resulting refractive index profile which is inherently periodic in its lower frequency component.
Periodic sampling allows the maximum refractive index change required for the grating to be reduced in comparison to non-optimised designs. Despite this, periodic sampling has several disadvantages. The periodic sampling design method yields a grating structure with spectral characteristics that are only approximately close to the desired multi-channel spectral characteristics. Depending on the particular multi-channel grating being designed, there may be significant deviations from the desired multi-channel spectral characteristics. Furthermore, periodic sampling cannot be used to directly design multi-channel gratings with non-identical group delay characteristics.
More recently, the design of multi-channel grating structures by dephasing reflection spectra in the spectral domain has been proposed. Using this design method, the desired reflection spectrum is described as a number of partial reflective spectra. The partial single spectra are then de-phased with respect to each other in the spectral domain to reduce the maximum refractive index variation to an optimal value. Out-of-band areas of the reflection spectrum can be suppressed or included depending on the design requirements. Inverse scattering analysis is used to reconstruct the profile of refractive index variation in the spatial domain from the de-phased partial reflection spectra.
The spectral dephasing design method overcomes some of the disadvantages associated with periodic sampling. Inverse scattering analysis achieves an exact translation from the spectral domain to the spatial domain. Consequently grating structures designed using this method possess a reflection spectrum which closely matches the desired reflection spectrum. In addition, it is possible to design multi-channel grating design with non-identical group-delay characteristics.
However spectral dephasing does not always result in an optimal maximum refractive index variation value. In many cases, spectral dephasing does not minimise the maximum refractive index variation to the extent possible. In particular, it has been found that the spectral dephasing design method does not result in an optimal maximum refractive index variation when designing multi-channel grating structures with channel to channel group delay which does not vary slowly or channel spacing which is not substantially equidistant.
FIGS. 5(a) to (d) show a multi-channel grating structure designed according to the process described in PCT/AU02/00307 which is assigned to the Applicant. FIGS. 6(a) to (d) show a multi-channel grating structure designed according to the process described in PCT/AU03/00959. As can be seen in both cases, the maximum refractive index variation of the grating structure is poorly optimised, to the extent that designs of the type illustrated in FIGS. 5(a) to (d) are impractical and difficult to achieve for most types of currently known grating fabrication techniques.
Reference to any background art in the specification is not, and should not be taken as, an acknowledgement, or suggestion, that this background art forms part of the common general knowledge in Australia or any other jurisdiction or that this background art could reasonably be expected to be ascertained, understood and regarded as relevant by a person skilled in the art.